Drug-containing polymeric fibers used for making stents are typically formed via co-compress molding, co-injection molding, or co-extrusion. For example, in a conventional co-extrusion process, one or more drug is compounded with a polymer resin. The compounded polymer resin is then melt extruded to form a fiber. However, the co-extrusion process has two important limitations. First, both compounding and melt extruding are carried out at the polymer's melting temperature, a temperature at which drugs are easily degraded. Second, the drug to polymer ratio must keep low in order to avoid breakage of the fiber during the melt extrusion process.
In fact, the drug to polymer ratio is critically important to the process of preparing drug-containing polymeric fibers by the above-mentioned techniques. It is routine in the art to keep the weight ratio of drug to polymer low so as to avoid severely compromising the mechanical strength of the polymeric fiber. Therefore, stents described in the prior art made using drug-containing polymeric fibers typically have a low drug content.
Polymer resins having a high- or ultra-high molecular weight are commonly used to make mechanically strong bioabsorbable polymeric fibers. Such strong fibers are desirable for use in manufacturing stents where mechanical strength is required. One drawback to using high molecular weight polymers is that the fibers made from them have a long degradation time. For example, a stent made from such fibers implanted in an artery will degrade slowly and may result in long term issues such as chronic foreign body reactions.
Another characteristic of existing drug-containing bioabsorbable fibers is a fixed drug release profile. Typically, upon implantation, a drug-delivery device formed from polymeric fibers initially releases a high concentration of drug that is rapidly reduced as the drug is released. In other words, the drug is present at a high concentration for only a short time. This drug release profile cannot be tailored to a specific disease condition.
There is a need to develop methods for producing drug-containing bioabsorbable polymeric fibers that overcome the problems associated with existing methods, e.g., low drug-loading capacity, manufacturing conditions that degrade the drug, long degradation time of the polymer, and non-programmable drug delivery profile.